This disclosure pertains to optical elements for use in interference lithography. In particular, this disclosure relates to a single reflective optical element that generates multiple beam interference where reflected beams are created by and are arranged symmetrically around a central incoming beam.
Photonic crystals (PhCs) are dielectric periodic materials with photonic bandgaps where the propagation of electromagnetic waves is forbidden. Studies of photonic crystals have been driven by their potential applications. One-dimensional PhCs such as fiber Bragg gratings can be fabricated easily for applications in fiber optical communications and fiber sensors. Two-dimensional PhCs can be used for an integrated laser on chip or all-optical circuit. Low threshold lasers in three-dimensional (3D) photonic crystals have been predicted and lasing oscillations have been observed in 3D PhC nanocavities with the highest quality factor yet achieved (˜38,500) with quantum dots. However a large-scale fabrication of 3D PhCs with large photonic bandgaps has been a challenge over the past decade. Several methods have been used for fabricating 3D PhCs, such as e-beam lithography for layer-by-layer structures, self-assembly of colloidal PhCs, two-photon direct laser writing, and laser holographic lithography.
Holographic lithography methods can produce 3D PhC templates by recording multi-beam 3D interference patterns in a positive or negative photoresists. So far holographic lithography has been successful in fabricating large-volume PhC templates at sub-micro/nano-scales. It is an adaptive method because the structure and symmetry of 3D PhC templates can be controlled by the beam propagating directions, the number of the interfering beams, the beam intensities, their respective polarizations and their relative phases. Among various structures, diamond-like and related woodpile structures have been intensively studied because of their wide and robust photonic bandgaps. However, the optical alignment is very complicated if bulky mirrors, polarizers, and beam splitters are used in multiple-beam holographic lithography. Very recently, a single diffractive or deflective optical element has been used for the laser holographic fabrication of 3D PhC structures in order to reduce the complexity of the optical setup and improve the optical stability. A flat-top prism and multi-layer phase mask have been demonstrated to fabricate diamond-like PhC templates by introducing a phase difference among the diffracted beams. Using a flat-top prism, a woodpile PhC template has been fabricated by introducing a phase shift π of a single side beam relative to others using a glass cover slip as a phase modulation. Other groups have demonstrated a realization of woodpile PhC templates by shifting two counter-propagating side beams by π/2 relative to the others using the prism. Although both methods have succeeded in the fabrication of a large scale 3D PhCs, the control of intensity ratios among the five beams was not considered. The flat-top prism has been used to overlap four linearly polarized side beams arranged symmetrically around a circularly polarized central beam. The polarization of each beam has been adjusted individually using wave plates mounted before the flat-top prism.
No known method has been developed for fabricating an integrated, single reflective optical element for interference lithography for the fabrication of PhCs. Such an optical element would eliminate the need to use bulk optics to control the laser polarization and to arrange the interfering laser beams, therefore greatly reducing the complexity of optical setups for interference lithography.